1. Field of the Invention
The present invention relates to an apparatus for decomposing waste of plastic products by heat to obtain useful fuel oil or gas.
2. Related Art
There have been proposed a large number of various apparatus for thermally decomposing waste of plastic products, i.e., polymer waste.
Japanese Laid-Open Patent Publication No. 9-268293 entitled "Catalytic Cracking Apparatus for Synthetic Polymers and Oily Material Production Process Using the Apparatus" discloses a technical idea of providing the apparatus with a screw extruder, a tubular pre-heater and a tubular decomposing furnace, feeding polymer waste by the screw extruder and melting it with heat applied thereto, then feeding the polymer melt material to the tubular pre-heater or the tubular decomposing furnace to be further heated so that polymers may be thermally decomposed and gasified, and condensing the resultant gas (decomposed gas) to obtain oily material.
The above publication also describes a structure that the tubular pre-heater and the tubular decomposing furnace are obliquely positioned, screws are arranged in the interiors of the screw extruder, tubular pre-heater and tubular decomposing furnace, to convey waste and a molten material thereof as well as to scrape off and automatically discharge sludge accumulated on the inner walls of the pipes, and the sludge is conveyed to the upper end of the tubular decomposing furnace by the screw therein and is dropped from the elevated position down to the outside of the decomposing furnace.
In addition, the publication describes that the apparatus is capable of continuous operation, and describes a feature that cracked gas generated in the upper portion of the tubular cracker is sealed by the polymer molten material stagnating in the lower portion of the cracker and thus is prevented from flowing back into the tubular pre-heater or the extruder on the upstream side.
However, there is no detailed explanation in the publication as to the structure for directly connecting the tubular cracker and the extruder for introducing raw materials, and concerning this structure, the publication does not disclose measures for preventing an undesirable situation where the gas in the tubular cracker flows back to the extruder side or measures for eliminating a possibility of explosion being caused by air entering the tubular cracker from the extruder. In principle, the disclosed apparatus uses a catalyst such as alumina. The publication describes an embodiment in which no catalyst is used, but in this case, no oily matter is obtained. Further, electric heat is used as a heat source for the tubular preheater and the tubular decomposing furnace. Although electric heat is easy to control and heat management is also easy, electric heat is an uneconomical heat source for this type of apparatus requiring a large quantity of heat.
In Japanese Patent Laid-Open Publication No. 10-17871, there is disclosed "Thermal Decomposition Apparatus for Polymer Waste" comprising a raw material melting tank, an inclined pipe for thermal decomposition and heating means, in which a screw feeder is arranged in the interior of the inclined pipe to convey molten polymer waste as well as to remove sludge, and the heating means is divided into first to third hot air furnaces for thermal decomposition so as to heat the inclined pipe in such a manner that the temperature thereof gradually increases from a lower portion to an upper portion.
This apparatus is capable of continuous operation and uses hot air as a heat source for primary and secondary decompositions, and therefore, a large quantity of heat can be supplied and oil produced by the apparatus itself can be economically used as fuel. However, the apparatus using the inclined pipe for thermal decomposition is constructed such that the inclined pipe extends through chambers (hot air spaces) for primary, secondary and tertiary thermal decompositions, each chamber occupying a large space, and thus an unnecessarily large heating space is required. Accordingly, a considerably large quantity of heat is consumed uselessly, and since the hysteresis of temperature change is large, improvement is still needed as to the difficulty of heat management. Also, the publication does not clearly set forth the relationship between the raw material melting tank and the inclined pipe for thermal decomposition with respect to an inflow of air and a backward flow of gas. Thus, the technical idea disclosed in the publication is not satisfactory in view of reduction in size of the apparatus and reduction in the operating cost.